1. Field of the Invention
This invention relates to coating compositions for applying antistatic layers to a support and to photographic imaging elements comprising an imaging layer and a support material containing an antistatic layer comprising a vanadium pentoxide colloidal gel dispersed in a binder. The invention is applicable to conventional photographic imaging elements as well as thermally processable imaging elements including thermographic and photothermographic imaging elements.
2. Description of Related Art
The problem of controlling static charge is well known. Static charging may occur due to various factors in the manufacture, finishing, and use of imaging elements, especially, photographic elements. The accumulation of static charges can result in fog patterns in photographic emulsions, various coating imperfections such as mottle patterns and repellency spots, dirt and dust attraction which may result in the formation of "pinholes" in processed films, and a variety of handling and conveyance problems.
To overcome the problem of accumulation of static charges, it is conventional practice to provide an antistatic layer in photographic elements. Many antistatic agents have been utilized for this purpose.
It is known to prepare an antistatic layer from a composition comprising a vanadium pentoxide colloidal gel as described, for example, in U.S. Pat. No. 4,203,769 issued May 20, 1980, U.S. Pat. No. 5,006,451 issued Apr. 9, 1991, and U.S. Pat. No. 5,221,598 issued Jun. 22, 1993. Antistatic layers containing vanadium pentoxide provide excellent protection against static and are highly advantageous in that they have excellent transparency and their performance is not significantly dependent on humidity. The excellent performance of these antistatic layers results from the particular morphology of this material. The colloidal vanadium pentoxide gel consists of entangled, high aspect ratio, flat ribbons about 50-100 angstroms wide, about 10 angstroms thick and about 1000-10000 angstroms long. Low surface resistivities can be obtained with very low vanadium pentoxide coverages as a result of this high aspect ratio morphology.
Typically, the vanadium pentoxide is coated in a polymeric binder to improve adhesion to adjacent layers and to improve the durability of the antistatic layer. Several polymer binders have been disclosed in the above-mentioned U.S. Patents, for example, interpolymers of vinylidene chloride for aqueous-based antistat coating formulations. As a result of the very low coverages required, the antistat coating formulations typically comprise concentrations for the vanadium pentoxide gel of about 0.1 weight % or less, typically less than 0.05 weight %. Such low concentrations result in coating formulations which are prone to instability and flocculation of the vanadium pentoxide gel. This creates serious difficulties in accumulation of flocculated vanadium pentoxide plugging) in coating solution delivery lines, filters, and coating hoppers. The addition of surfactant no the coating formulation to stabilize the vanadium pentoxide may help alleviate the problems of flocculation, but, this typically results in antistatic layers which comprise high levels of surfactant in the dried film. This is undesirable when the antistatic layer is to be overcoated with other layers such as protective overcoats or hydrophilic colloid layers such as gelatin-containing subbing layers, photographic emulsions, or curl control layers. The high concentration of surfactant in the dried antistatic layer can seriously affect the coatability and adherence of these subsequently applied layers.
Thus, it is highly desirable to provide an antistatic layer comprising colloidal vanadium pentoxide which is applied from a coating formulation with improved stability and has improved adherence to underlying and overlying layers. It is toward such objectives that the current invention is directed.